Site Selection for Barrage

Khadir:

Flood plan of river. Khadir axis is a line passing through the center of the river course between the two high banks up to back water effect.

Weir axis:

Line along which the crest of the weir is laid.

River axis:

Line parallel to Khadir axis at the center of weir or barrage axis between the abutments.

Headwork axis:

Line perpendicular to weir axis at the center of weir abutments

Guidelines proposed by the Irrigation Research Institute Lahore.

When the angle between the headwork axis and the river axis exceeds 10°, the problem arises of concentration of flow on one side and island formation due to heavy silting within the guide bank on the other side. If the river axis is to the right of headwork axis, the concentration of flow is generally generally on the left side with consequent tendency to form an island on the right side and vice versa.

Taunsa Barrage on River Indus

1. Should be located far below the influence of two river

2. Preferably located in the center of plain asymmetry result in shoals formation

Consideration for site of barrage:

1. For Good command along long feeder channels

2. Width of river at site should preferably be minimum

3. Good land approach

4. Foundation consideration

5. Existence of inundation canals - choice is limited if want to convert them into permanent canal

6. Easy diversion of river after construction

7. Existence of central approach of river to barrage after diversion.

Barrage width:

River in alluvial plains while in flood spread over miles in width and in dry weather flow in channels.

For optimum width Lacey's Equation, related to wetted perimeter to discharge wetted perimeter in case of shallow channel is almost equal to the bed width of the channel.

The barrage width must be sufficient to pass the design the flood safety. The present trend is to design barrage for a 100-150 years frequency flood. The minimum stable width of an alluvial channel is given by Regime Eq.

Regime or Scour Depth

Due to high flow, the river bed is scoured both on the upstream and downstream sides of the weir, large scour holes develop progressively adjacent to the concrete aprons, the weir foundations may slip into these scour holes, thus undermining the weir structure. The regime scour depth Rs may be estimated by following formula.

If actual waterway provided is greater or equal to the regime width and

If waterway provided is less than regime width and f = 1.75 under root d
d is mean diameter of bed material in mm.